In rapid thermal processing (RTP), workpieces such as semiconductor wafers can be subjected to specified temperature cycles of arbitrary complexity. For this reason, RTP is useful for carrying out thermally dependent processes, such as diffusion and annealing, in the course of manufacturing integrated circuits. However, some of these processes require the temperature to be controlled within limits as small as .+-.10.degree. C. or less. Such fine control is possible only if the wafer temperature can be measured to precision that is comparably high.
Optical pyrometry is one useful method for controlling the temperature of the wafer during RTP. One pyrometric technique has been described in U.S Pat. No. 5,154,512, issued to C. W. Schietinger et al. on Oct. 13, 1992. This technique is schematically illustrated in FIG. 1. According to this technique, a first light-pipe probe 10 is provided, having an input aperture that faces wafer 20, and a second probe 30 is provided having an aperture that faces one of opposing lamp banks 40, which are typically linear arrays of quartz-tungsten-iodine lamps situated outside of processing chamber 50. First probe 10 samples radiation emitted and reflected by the wafer and directs the sampled radiation into detector 60. Second probe 30 samples radiation emitted by the lamps and directs the sampled radiation into the detector. Probe 30 receives radiation both in a direct path from the lamps and also by reflection from reflector 80. The emissivity .epsilon. of the wafer is inferred from the probe signals, and then the wafer temperature is inferred from the Planck radiation law, which relates the wafer thermal emittance, the wafer emissivity, and the wafer temperature T.
As noted, the first probe signal is a sum of emitted and reflected radiation. Information sufficient to resolve the emitted and reflected components is available because the emission from the lamps, which are driven by alternating current, has an ac component referred to as "ripple". Because the thermal emission from the wafer has no significant ac component, the wafer reflectivity is estimated as the ratio of the ripple amplitudes in the first and second probe signals, respectively. After this reflectivity has been evaluated, the first probe signal is corrected to yield a resolved value of the water thermal emittance.
However, the above described techniques provide undesirable constraints on the placement of the probes. Because the ac signals of the probes are used to directly determine the reflectivity, one probe must "see" only the wafer and the other probe must "see" only the lamps. In the prior an pyrometer in FIG. 1, the light pipe probe 10 is configured and placed to see only the wafer 20 and the light pipe probe 30 is configured and placed to see only the light source 40. This provides certain constraints on probe placement. First, as illustratively in FIG. 1, the probes 10 and 30 project into the chamber. Particles are generated during the RTP process, and these particles deposit on the probes. Consequently, the probes must be removed and cleaned, which is a difficult and time-consuming process. Furthermore, since the process is sensitive to the position of the probe relative to the wafer and the lamp, the probes must be reinserted in almost exactly the same position they were in prior to removal. Otherwise, the pyrometer has to be recalibrated. Also because of the spatial constraints in the RTP enclosure 50, it is difficult to provide space for the probes 10 and 30.
Consequently, an optical pyrometry process and apparatus which does not have the above-noted constraints on probe placement is desired.
Although useful, this technique requires the probes to be located in proximity to the wafer or, at the very least within the oven enclosure. The probes are oriented such that they collect radiation incident upon, and reflected from, the wafer.